1. Field of the Invention
This invention relates to the field of controlling gear ratio changes in an automatic transmission. More particularly, it pertains to a technique for preventing powertrain hunting, i.e., gear hunting in such transmissions.
2. Description of the Prior Art
The invention relates generally to automotive transmission controls of the type disclosed in U.S. Pat. No. 4,943,921 and U.S. Pat. No. 5,016,175.
In an electronically controlled, variable speed automatic transmission, the current gear ratio is determined on the basis of a schedule relating vehicle speed to throttle position, the schedule being stored in electronic memory accessible to a microprocessor. The schedule defines boundaries between operating conditions defined by vehicle speed and throttle position, each boundary separating an operating range for a particular gear ratio from an adjacent range where a different gear ratio is the appropriate gear ratio. Generally, the shift schedule for each gear ratio produces a reference vehicle speed, the dependent variable, when memory is addressed with the current throttle position, the independent variable. The reference vehicle speed is compared to current vehicle speed periodically so that upshifts and downshifts from the current gear occur when vehicle speed rises above or falls below the vehicle speed at which a gear ratio change from the current gear is to occur according to the schedule.
However, when moving from a flat road to an inclined surface, vehicle speed declines if throttle position is maintained. Thereafter, in order to maintain vehicle speed, the throttle is opened by depressing an accelerator pedal. Typically, this action produces a downshift to a lower operating gear ratio where greater torque is delivered to the drive wheels of the vehicle. Consequently, vehicle speed increases. But if vehicle speed thereafter exceeds the desired vehicle speed, the operator partially closes the throttle, thereby inducing an upshift to a higher gear ratio where drive wheel torque decreases and vehicle speed drops.
Operation of the transmission in combination with the engine throttle in this manner is called "hunting", a cyclic repetition of gear shifts between adjacent gear ratios with corresponding changes in vehicle speed.
A similar condition can occur when moving from a flat road to a declining slope, vehicle speed can be maintained by partially releasing the accelerator pedal, but this usually produces an upshift to a higher gear ratio and an increase in vehicle speed.
Operating conditions other than a change in road grade can cause an automatic transmission to cycle or hunt between gear ratios. Changes in atmospheric pressure due to changes in altitude from that corresponding to the shift schedule; a substantial change in vehicle load from the gross weight corresponding to the shift schedule, such as when pulling a heavy trailer; degradation in engine condition; and a substantial elevated ambient temperature can induce powertrain hunting.
Various techniques have been devised to avoid this problem. For example, U.S. Pat. No. 4,335,418 describes a technique for detecting increased engine load when running on a slope and determining the appropriate gear ratio in accordance with the magnitude of the loading associated with the inclination of the slope. Slope detecting reference data, indicative of vehicle speeds for various ranges of slope correspond to throttle openings in different gear ratios, are used to detect presence of the slope by comparing the actual gear ratio, the throttle opening, vehicle speed, against the slope detecting reference data.
Signals indicative of the shift lever position, vehicle speed and throttle opening are applied as input to a microprocessor, a slope detecting program determines presence of the slope from these data and modifies standard shift data to produce shift reference data, which is then written into memory. By comparing the current gear ratio throttle opening and vehicle speed against corresponding reference data, a speed ratio is determined and used to energize solenoid valves to produce gear ratio changes in accordance with the control programs.
The gear shift control of U.S. Pat. No. 4,285,252 uses a stored schedule of vehicle acceleration and a comparison between actual vehicle acceleration to the stored value to change scheduled gear shift in accordance with differences between stored and actual vehicle acceleration. The control includes a first schedule of gear shift changes suitable when the vehicle is traveling on a flat road and a second schedule for gear changes suitable for an uphill road. On the basis of a stored schedule of vehicle accelerations, reference to throttle position and actual vehicle acceleration, the slope of the grade is determined, and a gear shift schedule corresponding to the slope is selected to control gear shifts while on the grade.
The gear shift control of U.S. Pat. No. 4,380,048 relies on a average value of engine load and vehicle speed to determine a programmed acceleration, which is compared to actual acceleration to determine whether a transmission shift characteristic is to be selected.
The gear shift control of U.S. Pat. No. 4,897,790 intentionally skips selected drive ratios on the basis of perceived driving conditions and modifies the scheduled gear ratio change on the basis of vehicle acceleration.